Waterproof plug



FIG. 1 is a perspective view of a waterproof plug showing my new design;

FIG. 2 is a side view thereof, the opposite side being substantially identical;

FIG. 3 is a top view thereof;

FIG. 4 is a bottom view thereof;

FIG. 5 is an end view thereof; and,

FIG. 6 is an opposite end view thereof.

The broken lines throughout the drawing are included for the purpose of illustrating environmental structure only and form no part of the claimed design. 

The ornamental design for a waterproof plug, as shown and described. 